1. Field of the Invention
The present invention relates to the purification of aqueous nitric solutions of rare earth values, said solutions containing, inter alia, alkaline earth metal cations, and more especially relates to the purification of such solutions by liquid/liquid extraction.
By the expression "rare earths" as utilized herein are intended the rare earth elements designated the lanthanides, having atomic numbers from 57 to 71, inclusive, as well as yttrium, having an atomic number of 39.
2. Description of the Prior Art
It is known to this art that the rare earth metals are typically found combined in different minerals, such as monazite, bastnaesite, xenotime, or various concentrates thereof; one means to separate the different rare earths from each other consists of carrying out successive liquid/liquid extractions beginning with aqueous solutions of said rare earths.
It is necessary that such starting aqueous salt solutions be of sufficient purity as to not adversely affect the course of the extraction operations. Similarly, the different rare earths separated cannot be used in certain applications if they contain more than a very small amount of impurities and of radioactivity.
Among the metals accompanying the rare earths, in aqueous solutions of the salts thereof, there are found, in particular, thorium, uranium and various metals such as alkali metals, alkaline earth metals, iron, manganese, titanium, and the like.
Among the alkaline earth metals, the presence of calcium, barium and radium, a product of the filiation of thorium Th.sup.232 and uranium U.sup.238 is noted.
Usually, in order to obtain high purity rare earth elements, they are separated from the other metals by extraction in an organic phase containing suitable extraction agent.
In view of the fact that the rare earths are present in preponderant amounts, it would be more convenient to eliminate the impurities consisting of the "other" metallic elements.
But even though it is known to preferentially extract, by liquid/liquid extraction, certain metallic impurities, in particular iron, the problem of separating the rare earths from the alkaline earth metals by extraction of the latter has not as yet been solved.